Actuators for influencing industrial processes are often designed such that their position is a function of a current fed into the actuator. Therefore, between a system for control of a process and the actuator of the process there are generally required members for conversion of the control signals, generated by the control system, into currents corresponding to these signals.
A fault in such a conversion member results in the capacity of the control system to control the process being reduced or lost, which may have serious economic consequences and which may also affect the safety. In those cases where the control functions are particularly critical, the conversion members are therefore often doubled, in which case normally only one of these members is connected to the actuator. When a malfunction is indicated in the connected conversion member, a switching is effected so that this member is disconnected and instead the other conversion member is connected. This switching is generally carried out with electromechanical relays, which, however, themselves contribute--to no negligible extent--to the rate of failure of those parts of the plants which are located between the control system and the actuator.